The fragrance or smell are substances produced by components in the food that are responsible for its organoleptic qualities. They mainly involve the sense of smell and taste. These substances determine, among other things, the sense of pleasure while eating. Although this may seem a trivial matter, the fragrance may be partially responsible for the evolutionary conditioning of the human being to distinguish between safe and dangerous foods. In this manner, sweetness is one of the most accepted tastes in humans and the first to natively develop, being the taste of breast milk. Sourness, on the other hand, is generally associated to inadequate or toxic foods. This is the case of the alkaloids, which are present in many toxic plants.
The taste of food implies the activation of the taste buds and olfactory components, through volatile elements and gases, which reach the nose through the oral cavity. Even though there are components capable of activating both the sense of smell and taste, there are other sensations that complete the sensorial properties of the foods, such as mechanic or thermic sensations. Therefore, the substances that produce the fragrance and taste are varied.
The scientific research of tastes and fragrance is not as precise as for example colour, since there is no specialized apparatus that is capable of measuring or giving a numeric value to them. Therefore, organoleptic tests are performed, on populations of consumers or by experts (tasters), to determine its quality. Even its chemical analysis is not capable of determining the components responsible for the taste and fragrance, since many substances are under the range of detection. On the other hand, the human sense of smell is very sensitive. For example, the detection of 2-methoxi-3-hexilpyrazine is 1×10-6 ppm (responsible for the smell of toast), in other words, we are capable of detecting 1 mg of the substance in 1 Tm. This sensitivity is even more remarked in many animals.
There are 4 basic tastes: sweet, salty, bitter and sour. If the fragrance is considered the taste becomes more complex, and can be spicy, fruity, meat-flavoured, etc. The fragrance is many due to substances found in low concentrations in the food (10-15 mg/kg of food), but of a large variety. Of these, certain fragrances give the characteristic taste and smell of specific foods, and are called “components with an impact character.”
The origin of fragrance or taste can be natural, in other words, is derived from the biosynthetic reactions that occur in the food. This is the case, for example, of edible plants. The fragrance and taste can also be due to chemical reactions that occur during its processing, such as the Maillard reaction that forms pyrroles, pyrazines, oxazoles, pyrrolines, pyrrolidines, pyrenes, reductones, thiazoles, and thiazolines. All these substances are formed during more or less intense industrial thermal and culinary treatments, giving the tastes of toast, burnt, baked, etc. The heterocyclic polysulphurs and other sulphur derivatives are common in cooked meats. Next, the main tastes and fragrances of foods will be commented in more detail:
Sweet smell/fragrance: This is generally due to the presence of sugars. In this sense, sucrose is the reference for level of sweetness. However, there are sugars with little or no sweetness.
Salty smell/fragrance: This is generally detected by the outer border of the tongue and is due to inorganic salts present in the food, mainly NaCl (sodium chloride) and to a less extent KCl (potassium chloride) and CaCl2 (calcium chloride). In fact, Na+ (sodium ion) from the NaCl and adding table salt to the food (mainly meats and fish) is the most common reason for the salty taste.
Acidic taste/fragrance: This taste is always associated to foods with low pH. In this case, the protons (H+ ions) play an important role in the taste, but is not the only reason. It is possible to sense an acidic taste when an electric current is passed through the mouth, such as when placing the tongue on one of the poles of a battery, since this generates protons. The components of the acidic tastes are organic acids, present in fruits and vegetables, such as malic or citric acid (in citrus fruits), tartaric acid (grapes), isocitric (blackberries), and oxalic (rhubarb), among others. When the taste buds that detect the acidic taste are saturated, it changes towards a bitter and sour taste. This occurs with acetic acid (vinegar), where its high concentrations (10-15%) provides a bitter taste to vinegar and products that are combined with it, such as coleslaw and pickles. Another possible product is lactic acid, derived from lactose fermentation. This is the main reason for the smell in cheeses. Coffee presents many substances that give it an acidic flavour. These are phenolic acids of the ferulic, caffeic and chlorogenic types. However, in this case the predominant flavour is the sourness that appears during its toasting, such as thiosubstitute furanes or caffeine, as well as other substances such pirrolic compounds derived from trigonelline.
Bitter taste/fragrance: This is detected by the taste buds located in the posterior part of the tongue. It is possible that this taste was developed during evolution in order to protect humans from consuming certain plants rich in toxic alkaloids such as nicotine, emetine, and atropine. Many of these alkaloids are used in pharmaceutical drugs. The tastes is due to inorganic salts such as KBr (potassium bromide) and KI (potassium iodine), as well as phenolic substances. With respect to the latter, they are very abundant in the peel of citric fruits such as oranginine and limonine, which can cause taste problems when producing juice from these fruits as they can be transferred in large quantities to the product. In beer products, on the other hand, the bitter taste is an important organoleptic requisite. It appears by adding hop (Humulus lupulus), a flower from a plant that provide resins which give the bitter taste to beer. Another example is tonic water and certain cold vermouths, where quinine alkaloid is added. The extracts from amino-acids and peptides resulting from the hydrolysis of lactic serum proteins or from animal blood also have a bitter taste. This taste is due to the abundance of hydrophobic aminoacids (valine, leucine, isoleucine, phenylalanine, tryptophan and tyrosine). For this reason casein and soy proteins possess a bitter taste.
Astringency: This flavour corresponds to the sensation of dryness associated with bitterness, which not only affects the tongue but the whole oral cavity. It is a desirable sensation for certain beverages, such as red wine, tea or cider. It is associated with the presence of tannin-type polyphenols. The most widely studied foods are wines and black tea. Specifically, the substances responsible for the astringency in black tea has been identified, such as epigallocatechin gallate. In fact, epigallocatechin is bitter, but gallate makes it astringent. As for the fragrance, other polyphenols are involved, which are formed during the fermenting of the leaves through the effect of catechol oxidase. This enzyme oxidizes flavonoid substances (which comprise 15-25% of dry weight of tea), such as catechines and derivatives. The result of the reaction are o-quinones, which gives rise to theaflavin (dimer)) or thearubigin (polymer) when condensed, conferring its astringency. Orange-red teaflavins also contribute to this flavour.
Pungency or spicy flavour: This flavour is also noted in the whole oral cavity. It is due to substances that are abundant in Cruciferous plants, particularly the Capsicum gender. The flavour is due to capsaicinoids, such as capsaicin and di-hydrocapsaicin, which are the base for chilli peppers and spicy chilli. The white and black peppers are from the Piper nigrum plant, whose main substance is piperine. Ginger (Zingiber officinale) is found in gingeroles and sogaholes, whose structure is similar to capsaicin and piperine. Other groups of spicy vegetables include wild radish (Amoracia lapathiofolia), white and yellow mustards (Brassica nigrum, Brassica alba) y radish (Taphanus sativus). The raw tissues of other vegetables are also spicy, such as those from the Brassica gender (cabbage, Brussel sprouts and curly kale). These plants present an enzyme called mirosinase, which is released when the plant is damaged mechanically. It catalyses the degradation of glucosinolates into isothiocianate compounds, which confer the spicy flavour. However, when these plants are boiled, the enzyme is inactivated and sulphur compounds are produced, giving rise to the characteristic smell of cooked leafy vegetables. Garlic and onion presents a characteristic spicy flavour due to the presence of sulfoxides derived from S-alkil-cysteine, such as alliin (in garlic) or propenil (onion). Other compounds include allicin, various sulphurs, thiophene, and thiosulfonates. Many of the spicy compunds are volatile and are released when sliced, producing irritation of the mucosa, causing the characteristic lacrimation of the manipulator.
Meat taste: Also known as the Japanese word “umami.” The substances responsible for this taste are varied, however Japanese cooks discovered two substances found in seafood. One is inosine monophosphate, isolated from dehydrated fish, and monosodium glutamate, from algae. These substances must be present in high concentrations in order to give food a meaty taste. However, when placed together, they work synergistically, becoming 20 times more potent than when used individually. For this reason monosodium glutamate is used as a flavour enhancer in dried soups, as long as inosine monophosphate is present in the food. In meats, glutamate is derived from the protein degradation and inosine monophosphate from the degradation of ATP. However, meats present many umami compounds, such as dipeptide carnosine. Other substances also appear when cooked, due to the denaturalization of the aminoacids or oxidation of fatty acids, or when they are combined with sauces or garnish.
Fruit flavour: This comprises the mixture of sweet flavours from sugars (glucose, fructose, sucrose) and bitter taste of organic acids (citric and malic acids). In addition, the fragrance of many volatile compounds also give the characteristic fragrance of many fruits, such as strawberries and citreous fruits. In the citreous fruits, the terpenoids predominate, whereas in other fruits such as apple or banana, the main compounds are aliphatic acids, alcohols, esters, aldehydes, ketones, acetates, terpenoids, and more than 120 other compounds. In addition, other compounds appear when the fruit mature, further contributing to its fragrance. On source of these maturing essence are the result of lipid oxidation, such as 2-nonenal. Another source are the aldehydes that appear from the decarboxylation and transamination of free amino acids. (such as leucine which gives rise to 3-methyl butanal). However, although the fruit fragrance seems to be very complex due to the large number of substances involved, in reality the fragrance is due to only a small number of them. For example, isopentenile acetate is very important for the banana´s smell, while eugenol and cinnamic aldehyde are abundant in cinnamon (which is not a fruit but the bark of the Cinnamomum tree). In cherries, the substance responsible for their fragrance is benzaldehyde, whereas in raspberry its 1-(p-hydroxyphenyl)-3-butanone. Generally, the smells of green, immature fruits are due to hexanal and 2-hexanal (very abundant in green apples), and the smell of mature fruit is due to cis-3-hexenol and a and b-ionone. In the case of citreous fruits, the terpenoids (which give rise to its smell) is mainly found in the skin, and can pass to the juice depending on how it is extracted. This compound is an oleos substance that can be isolated by distillation and is considered an essential oil. These oils include limonene and citral (mixture of geranial and neral), which are abundant in lemons, oranges and grapefruit.